ossible suspects in the demise of the Mediterranean diet are not hard to find in the food court of Plenilunio, a giant mall not far from Madrid airport that offers customers 138 shops, a multiscreen cinema and dozens of restaurants.

If visitors are not in the mood for a McDonald’s, Burger King or Subway, there’s a KFC, a kebab restaurant, a noodle place, a sandwich bar, a tex-mex joint, a US-style diner or two Italian chains. Steak lovers can choose between Argentinian, Brazilian or American options, while a lone outlet meekly peddles “healthy Asian food”.

Conspicuous by their absence – barring a couple of tapas restaurants – are places offering the kind of traditional Spanish food that forms part of the celebrated Mediterranean diet.

This week the World Health Organization said the fabled healthy way of eating was dead, as far as children in Spain, Italy and Greece were concerned. Fruit, vegetables, fish and olive oil had given way to sweets, fizzy drinks and junk food, leaving more than 40% of nine-year-olds in those countries overweight or obese.

“The Mediterranean diet for the children in these countries is gone,” said Dr João Breda, head of the WHO’s European office for prevention and control of noncommunicable diseases. “There is no Mediterranean diet any more.”

Breda’s lament met with a little scepticism from María Reguera, who was sitting in the food court at Plenilunio with her parents and three-year-old daughter. “I don’t think it’s so much of a problem because if you look at the nurseries and schools, they teach children about good habits like eating fruit and vegetables,” she said. “People just use places like this at the weekends or for after-school snacks. I still cook at home and I’m not worried.”

Her mother, Cristina Rojo, said social changes had brought about a break with past conventions. “I think the problem is that young people work nowadays and they have less time to cook,” she said. “Before, women weren’t working and they had more time to cook. People just have less time now. They just do what’s easiest.”

Despite that, said Rojo, home-cooked weekend lunches were still a sacred part of Spanish life.

Experts in Italy argue that the death knell for the Mediterranean diet has been sounding for decades. Antonino De Lorenzo, a diet and nutrition professor at the University of Rome Tor Vergata, traces the changes back to the early 1980s when low-cost food manufacturing and access to junk food in Italy became more widespread.

“Only 10% of Italians truly adhere to a Mediterranean diet,” he said. “The erosion of the diet is also influenced by the media, television and the promotion of food that is low quality. We have a big fight to try to change this.”

He said that as a result of an unhealthier food intake, Italian children aged eight to 11 were now “more equal to their American counterparts” in terms of weight levels.

Authorities in Italy have moved to ban new kebab shops and places selling other fast food. Photograph: Alamy

Protests in the last few decades against the proliferation of fast-food outlets in Italian cities have proved unsuccessful. However, authorities in cities such as Venice, Verona and Florence have more recently moved to ban the opening of new kebab shops, places selling pizzas by the slice, and other fast food.

Nutrition specialists and officials in Greece and Cyprus also see the WHO’s findings as evidence of the rapid lifestyle changes across the eastern Mediterranean. Antonia Trichopoulou, who formerly headed the Federation of the European Nutrition Societies, said it was clear that a diet developed over the millennia “as a natural experiment” in Greece had fallen victim to marketing prowess.

“Olive oil began to be abandoned in the 60s and 70s when there was a huge marketing of seed oil,” she said. “Then margarine was introduced. There was no need for the Mediterranean to have margarine because it had never been a high consumer of butter.” In the 60s, southern Europeans also began eating meat in abundance and a lot of cheese.

Authorities in Cyprus have begun taking action. Programmes are being established in schools, with the government inaugurating free distribution of fruit, vegetables and milk at kindergarten and elementary level.

A mobile food shop in Famagusta, northern Cyprus Photograph: Alamy

The €870,000 campaign, mostly funded by the EU, is aimed not only at changing the eating habits of children – and weaning them off sugary soft drinks – but altering their attitudes towards fresh food and milk more generally.

Officials are mindful that without intervention, the long-term effects could be explosive. “Societies like ours have changed radically in two or three generations and suddenly small communities have access to fast food with all the negative health impacts we are seeing,” said a Cypriot government spokesman, Prodromos Prodromou. “We have to act now.”

Trichopoulou, who has been studying the Mediterranean diet since the 80s and is regarded as Europe’s pre-eminent expert in the field, said while lack of exercise and bad eating habits were clearly to blame for child obesity, all was not lost.

“I am optimistic that the Mediterranean diet will not die in the next generation,” she said. “There has been a lot of movement in recent years to rediscover traditional foods in Greece, with people moving to villages and producing local food. The Mediterranean diet is seen as sustainable and good for the environment. It can be recovered.”

At the Plenilunio food court, close to the cinema entrance, a family sat unwrapping their dinner. “It’s hard for me to talk about the traditional Spanish diet as we’ve only been here for a year,” said a Venezuelan man as his sons tucked into Burger King kids’ meals. “But we are eating a lot of hamburgers.”